The present invention relates to drying apparatae and methods. The invention finds particular application in connection with cylindrical dryers for use in the Fourdrinier paper making process and will be described with particular reference thereto. It is to be appreciated, however, that the present invention will find utility in connection with other drying and heating processes for particulates, other sheet-like goods, and other materials.
Heretofore, steam has been the primary energy source for drying paper. In a typical paper manufacturing process, 90 to 300 pounds of water are removed for each pound of paper produced. Significant fractions of the water are removed mechanically in a forming section as the paper is formed, and in a pressing section as the paper is squeezed between rollers. In the pressing section, the cost of dewatering the paper is about forty times as much per gallon as in the forming section. Commonly, the paper leaves the pressing section with a dryness of about 40%, wet basis. To dry the paper to a 92-95% dryness required for the finished product, another 1 to 2 pounds of water per pound of paper are removed in a drying section. Conventionally, the drying section included a plurality of steam heated rollers around which the paper web passed. The cost per gallon of water removed in the dryer section was typically about 80 times the cost per gallon in the forming section.
The Fourdrinier paper making process is suitable for manufacturing paper of varying grades. Heavier grades of paper, such as linerboard or paper board, require more energy per linear foot during the drying stage than lighter grades. Commonly, steam heated drying cylinders which are sized for lighter grades of paper achieve too low a thermal head to dry the heavier paper grades at full, normal production rates. To dry heavier grades, the manufacturing rate of the entire system must be reduced.
Additional thermal energy cannot be obtained from steam dryers because the maximum pressure for these dryers is limited to 160 psig by the ASME Unfired Pressure Vessel Code. This achieves a saturated temperature of about 370.degree. F. Adding steam cylinders and increasing the corresponding steam production to handle heavier paper grades adds considerable production expense. Moreover, adding additional steam drying rollers, which are commonly 5-6 feet or more in diameter, requires a significant lengthening of the manufacturing line and, in many instances, of the physical facilities necessary to house the line.
The present invention overcomes the above-noted problems and disadvantages, and provides an improved high thermal head dryer which increases drying capacity, provides better performance, and is adaptable to effective use with a wide range of paper grades.